This invention is related generally to a directional ground relay system, and more particularly to such a relay system for protecting electric power transmission system against high-resistance ground fault.
A high-resistance ground fault is generally detected by the phase relation between zero-phase sequential current and zero-phase sequential voltage as described in Electric Technology Research Association Report Volume 37-1, 1981, p. 49, 50 and 54, the entire content of which is incorporated herein by reference.
A directional ground relay of this type has a forward ground directional detecting element and a reverse ground directional detecting element. The directional ground relay sets zero-phase sequential voltage Vo as reference, and decides that there is a forward ground fault if the zero-phase sequential current Io lags −Vo, and that there is a reverse ground fault if the zero-phase sequential current Io leads to −Vo.
FIG. 21 shows the characteristic sample of a typical directional ground relay described above. In FIG. 21, “θ” denotes the highest sensitivity angle of the directional ground relay.
FIGS. 22 and 23 show examples of logic sequence circuits for tripping the circuit breaker using the outputs of the directional ground relay having a characteristics shown in FIG. 21, especially, the ground fault forward detecting element 21 which may detect a forward fault. FIG. 22 shows a case when the local circuit breaker is tripped. The logic sequence shown in FIG. 22 has a timer 12 (on-delay timer) to delay the operational output of the ground fault forward detecting element 21 for confirming the operation. Thus, the directional ground relay may be used as a backup relay. The numeral “13” denotes the trip command output signals for phases A, B and C.
FIG. 23 shows another example of a logic sequence circuit which makes a quick trip when the ground fault forward detecting element 21 is activated and a permission signal is received. In FIG. 23, the permission signal is received by a permission signal receiving unit 14, and the output of the unit 14 and the output of the timer 12 are received by an “AND” gate circuit 36. The output of the “AND” gate circuit 36 is sent out as the trip command output signal for each phase 13. The output of the timer 12 is sent to a remote terminal via a permission signal sending unit 18.
When the directional ground relay shown in FIGS. 22 or 23 is used for tripping, the setting time of the delay timer 12 for operational confirmation is set to zero second. When the directional ground relay is used as a backup relay, the setting time of the delay timer 12 is typically set to hundreds mili-seconds to several seconds.
The directional ground relay described above utilizes zero-phase sequential current Io and zero-phase sequential voltage Vo. It may decide the fault direction even at a high-resistance ground fault, but it may not decide the faulty phase. However, since the fault current is small at a high-resistance ground fault, tripping all phases including healthy phases at same time as the faulty phase would affect the whole power system more severely than letting the fault continuing. Thus, when a circuit breaker is tipped for a high-resistance ground fault, only the faulty phase should be tripped. Besides, when the ground resistance is very high, zero-phase sequential voltage Vo may be very small, and the fault direction may not be decided.